Synaptic and Cellular Organization of Layer 1 of the Developing Rat Somatosensory Cortex

In this thesis, we systematically characterised the circuitry, cellular composition and synaptic connectivity of layer 1 (L1) of the somatosensory cortex of juvenile rats (post natal days 13-16). Electrophysiological and morphological parameters were measured using single and multi-cell patch-clamp electrophysiology followed by histochemical staining and 3D morphological reconstruction. Guided by the Petilla convention, we classified cells in term of their firing patterns, identifying five electrophysiological types: classical Non-Accommodating Cells (cNAC), classical Accommodating Cells (cAC), burst Non-Accommodating Cells (bNAC), classical Stuttering cells (cSTUT) and classical Irregular Spiking cells (cIR). We created first a subjective and then an objective classification of different morphological cell types. The subjective classification identified the following types – NGC-DA, NGC-SA, HAC, DAC, LAC and SAC. The initial classification was refined and validated using PCA and LDA. We also studied connectivity patterns among L1 cells. Our results showed that a majority of the cells were connected by a slow GABAB mediated inhibitory connection, with a fast GABAA component. Multiple repetitions of stimulation produced a prominent run-down of postsynaptic potential amplitudes, which was not seen in perforated patch-clamp recordings that maintained the intracellular milieu. We propose that the run-down was due to the dilution of intracellular components and depletion of secondary messengers necessary for signalling of postsynaptic metabotropic GABAB receptor mediated responses. 3D morphological reconstructions of these pairs of cells, showed the presence of multi-synapse connections with an average of 9 putative contacts per detected electrophysiological connection. The average coupling co-efficient of gap junctions was 0.054 ± 0.03 with symmetrical conductance. In additional studies, we investigated patterns of inter-laminar connectivity, the modulation of apical dendritic activity by L1 neurons and patterns of gene expression in L1. We hope that the results these studies – single cell electrophysiology, morphology and connectivity analysis - provide useful hints for future studies of neocortical L1.